About the down arrow, I have no idea. It only started to do it today. It was normally the up arrow that would give me problems
About heel/toe: The majority of the time that I play, I play flat-footed. I found playing this way years ago to use less energy than playing on my toes. I also exercise here and there, so that helps some. Stamina is a huge deal when it comes to dance games. There used to be days where I can barely finish Max 300 or Maxx Unlimited, but now they're cake. Build stamina by going for jogs/long bike rides or something.
Also, do you have a local arcade that has regulars that play? In my early days, I got a lot better by just watching others play.
The next time I go, I'll try and get some videos of my body to show you how I play.